Mexican telecom magnate Carlos Slim lambasted the Organization for Economic Cooperation and Development on Tuesday for saying that a lack of competition in the telecommunication industry costs the Mexican economy $25 billion a year.

"We don't know where they got that figure," Slim said, calling it a "fantasy." The billionaire also questioned the model that the OECD used to reach the estimate.

The OECD singled out Carlos Slim's operators in a report late Monday, noting that America Movil SAB (AMX) claims about 70% of the Mexican wireless market while its fixed-line unit, Telmex, has about 80% of land lines. The OECD said that the average market share of the mobile incumbent in OECD countries is roughly 40%.

The OECD argued that a lack of competition in Mexico's telecommunications market has led to "extremely high prices" for consumers and businesses, and slowed the take-up of new services. The OECD estimates the cost of muted competition to the Mexican economy to be around $25 billion each year, equivalent to nearly 2% of Mexico's gross domestic product.

Slim took issue with the assertion that Mexicans pay a high price for phone service, citing a December Bank of America Merrill Lynch report that shows Mexico's revenue per minute from mobile users almost on par with that of the U.S, at $0.04 versus $0.03.

Over the years, Slim said, America Movil's Mexican mobile brand, Telcel, has been reducing prices to consumers while increasing minutes-of-use. Fixed-line operator Telmex hasn't raised rates in years.